1. Field of the Invention
This invention relates to apparatus for cold forming metals, and particularly to pilgering apparatus for reducing the cross-section of tubing such as zirconium alloy tubing used in making fuel rods for nuclear power plants.
2. Background Information
A pilger mill is a forming apparatus for axially elongating and reducing the diameter of a ductile tube. A pair of opposed rollers or dies having tapered grooves around a portion of their circumference bear on the tube which is supported on a mandrel. The dies reduce the external diameter while the mandrel prevents the tube from collapsing under the force of the roller dies and dictates the inner diameter of the tube. The dies are operated in a cyclic manner to incrementally cold work the tube. With each stroke, the tube is advanced along its longitudinal axis and rotated.
Pilger forming is used in the production of precision tubing such as zirconium alloy tubes used in cladding nuclear fuel. The machine operates at up to about 250 work and return stroke cycles per minute.
Conventionally, the dies are ring shaped and are heat shrunk onto a shaft or arbor which is journalled at both ends and geared for coordinated rotation. Typically, the annular die is provided with a keyway engaged by a key on the shaft or arbor for synchronization of the timing of the confronting die pair.
Changing of such a conventional pilger die requires removal of the die and arbor weighing several hundred pounds from the pilger machine using a crane. The spent die must then be heated to remove it from the shaft before a new die can be heat shrunk in its place. This is a time consuming practice averaging well above three hours.
An attempt was made to reduce the change out time for pilger dies by using a die with a horseshoe shaped opening which allowed it to be laterally slid on and off the arbor. The die was fixed in place on the arbor by axial clamping members. This procedure eliminated the need to heat shrink the die onto the arbor and to remove the arbor from the pilger machine saddle, resulting in a very substantial reduction in the time required. However, the reciprocation of the arbors and dies at 250 or more cycles per minute generated forces which made it difficult to maintain the die on the arbor.